1. Field of Invention
This invention pertains to rotating drums which have magnetic heads mounted thereon and which are used in helical scan tape drives, and particularly to a method for determining and verifying parameters of such drums.
2. Related Art and Other Considerations
In a helical scan tape drive, traveling magnetic tape is at least partially wrapped around a rotating drum (or scanner) so that heads mounted on the drum can transduce information to and from the tape in a format of helical stripes. The drum has at least one write or recording head mounted thereon, and also at least one read head.
On a drum of a helical scan drive, one head is distanced from another head both by a radial distance and an axial distance. The axial distance is taken along the major axis of the drum, the radial distance is an angle about the major axis of the drum. The separation of two heads along the major axis of the drum is herein denoted as the "vertical offset distance" or "VOD", or alternatively as the "axial offset distance" or "AOD".
In the manufacturing of a drum for a helical scan recorder, it is desired that the axial offset distance be properly set. Historically, drum or scanner manufacturers have used high-power optical microscope measurement systems to adjust and verify the spatial alignment of the heads of a drum during production. This optical technique has several disadvantages which have become more significant as track densities have increased.
A first disadvantage stems from the fact that the optical measurements used to establish the spatial head positions are made from the physical edges of the magnetic materials that form a front gap area of each head. This measurement is actually an approximation since the effective "magnetic" edges of the head gap (which interact with the magnetic media either writing or reading) do not necessarily coincide with the physical material edges on the tape- rubbing surface of the head. The difference between the physical and magnetic edges are typically small, on the order of 1 .mu.m, which could be ignored at low track densities. However, as track densities have now increased to greater than 80 tracks/mm, the head spatial positioning errors due to the difference between the physical edges and the magnetic edges can be significant (-10% of track pitch) and thus cannot be ignored.
As a second disadvantage, the spatial optical measurements are made with the drum held stationary, i.e., the drum is not rotating. However, the high-speed dynamic head plane of the scanner (as used during writing or reading) may differ from the low-speed static head plane (as measured optically). Although the differences between the high-speed dynamic and low-speed static head planes are also small (on the order of 0.5 .mu.m), as track widths narrow (e.g., approach 10 .mu.m or less) the difference becomes a more significant factor.
As a third disadvantage, interactions of very small imperfections in the scanner spindle components (for example, the ball-bearing defects) cause a non-repetitive head plane error (a.k.a. non-repetitive runout or NRR) that can be accounted for only if many (say several hundred) measurements are averaged. This is not practical for the optical measurement technique (for even relatively low- volume production) since each complete optical measurement of a multi-headed scanner might take several minutes. Accordingly, drum manufacturers have historically ignored NRR.
A fourth manufacturing disadvantage for the optical measurement technique is that such optical measurements are only practical with the drum removed from the tape drive and placed in special tooling. This means that the spatial head positions cannot be verified easily as a routine part of final tape drive testing.
U.S. Pat. No. 5,291,134 to Magnusson, incorporated herein by reference, describes an electrical measurement technique that can be used to determine and/or adjust the location of the magnetic flux emanating from the erase head gaps of a scanner relative to the magnetic pattern previously recorded by the write (or "record") head gaps of the scanner. The method described in U.S. Pat. No. 5,291,134 is only a relative technique in that it merely determines how a second magnetic flux pattern from the erase head gaps interacts with a first magnetic pattern previously recorded by the write head gaps. The method of U.S. Pat. No. 5,291,134 gives no absolute information about the first magnetic pattern recorded by the write head gaps. Moreover, in the technique described by U.S. Pat. No. 5,291,134, both the first magnetic pattern from the write head gaps and the second magnetic pattern from the erase head gaps are recorded with a constant (native) ratio between the scanner RPM and the linear tape speed.
U.S. Pat. No. 5,731,921 to by Hughes, entitled "Method and Apparatus For Determining And Using Head Parameters In A Helical Scan Recorder", incorporated herein by reference, provides methods of determining an axial offset variance (AOV) of a write head with respect to a read head. The axial offset variance is a differential between a desired (e.g., specification mandated) axial offset distance and an actual axial offset distance by which the heads are actually separated on the drum. The methods disclosed in U.S. patent application Ser. No. 08/561,155 for determining axial offset variance involve transporting the tape past the drum at a constant linear velocity.
What is needed therefore, and an object of the present invention, is a non-microscopic technique for properly determining the axial offset distance between two heads on a rotating drum of a helical scan tape drive.